U.S. patent number 6,394,363 [Application Number 09/673,384] was granted by the patent office on 2002-05-28 for liquid projection apparatus.
This patent grant is currently assigned to The Technology Partnership PLC. Invention is credited to Michael George Arnott, Victor Carey Humberstone, Simon Roger Johnson, Richard Wilhelm Janse Van Rensburg.
United States Patent |
6,394,363 |
Arnott , et al. |
May 28, 2002 |
Liquid projection apparatus
Abstract
The present invention relates to liquid projection apparatus in
the form of a face-shooter array. Material layers are used as the
basis to fabricate the device to overcome constructional
difficulties associated with other technologies. The device
utilizes excitation of the surface layers (100) incorporating
nozzles (8) which are arranged over one surface layer with
addressability, forming a liquid projection array, capable of
operation at high frequencies with a wide range of liquids.
Inventors: |
Arnott; Michael George
(Cambridgeshire, GB), Johnson; Simon Roger
(Cambridgeshire, GB), Humberstone; Victor Carey
(Cambridgeshire, GB), Van Rensburg; Richard Wilhelm
Janse (Cambridgeshire, GB) |
Assignee: |
The Technology Partnership PLC
(Rayston, GB)
|
Family
ID: |
10830511 |
Appl.
No.: |
09/673,384 |
Filed: |
October 17, 2000 |
PCT
Filed: |
April 16, 1999 |
PCT No.: |
PCT/GB99/01164 |
371(c)(1),(2),(4) Date: |
October 17, 2000 |
PCT
Pub. No.: |
WO99/54140 |
PCT
Pub. Date: |
October 28, 1999 |
Foreign Application Priority Data
|
|
|
|
|
Aug 17, 1998 [GB] |
|
|
9808182 |
|
Current U.S.
Class: |
239/102.1;
239/102.2; 239/553.3 |
Current CPC
Class: |
B41J
2/14201 (20130101); B05B 17/0607 (20130101); B41J
2202/15 (20130101); B41J 2202/20 (20130101) |
Current International
Class: |
B05B
17/04 (20060101); B05B 17/06 (20060101); B41J
2/14 (20060101); B05B 001/08 (); B05B 003/04 ();
B05B 001/14 (); F23D 014/68 () |
Field of
Search: |
;239/102.1,102.2,304,305,438,456,536,549,551,554,555,568,553.3
;347/46,47,68,69,70 ;134/1.3,2,3,26,28 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Yuen; Henry C.
Assistant Examiner: Hwu; Davis
Attorney, Agent or Firm: Dykame Gossett PLLC
Claims
What is claimed is:
1. A device for projecting liquid as jets or droplets from multiple
nozzles, the device comprising:
a plurality of transducers oriented substantially parallel to one
another and each having an inner face and an outer face opposite
said inner face, the transducers being arranged in a substantially
planar array;
a plurality of nozzles, each nozzle being associated with a
respective transducer and having corresponding inner and outer
faces and an axis normal to said inner and outer faces of said
transducer, said transducer being excitable to cause movement of
the associated nozzle in a direction substantially aligned with the
nozzle axis, to project liquid therefrom;
liquid supply means for supplying a liquid to said inner face of
said nozzles;
means for selectively exciting transducers as required, thereby to
project liquid as jets or droplets from the respective outer face
by movement of the liquid through the nozzle in response to the
movement of the nozzle.
2. A device according to claim 1, wherein the transducers are
formed in regions of a material layer having an outer face and an
inner face, at least some of said regions having nozzle-bearing
sub-regions through which nozzles extend from the inner face to the
outer face, and including
a multiplicity of excitation means, each capable of exciting at
least one of said regions and/or nozzle-bearing sub-region.
3. A device according to claim 2, wherein said regions are in the
form of beams formed by slits within said material layer.
4. A device according to claim 3, wherein each of the slits is
sealed.
5. A device according to claim 3, wherein said slits are arranged
in the form of a comb.
6. A device according to claim 5, wherein the slits are arranged in
the form of two interdigitated combs.
7. A device according to claim 2, wherein terminals for supplying
actuating signals to the transducers are formed in the material
layer in which the transducers are formed.
8. A device according to claim 2, wherein a region of the material
layer adjacent each transducer is thinner than other parts of the
material layer.
9. A device according to claim 1, wherein the transducers are
formed in regions of a first material layer which contains a
plurality of apertures therethrough,
the nozzles are formed in sub-regions of a second material layer in
registry with the apertures in the first material layer; and
including
a multiplicity of excitation means, each capable of exciting said
second material layer directly or indirectly in the sub-region of
at least some of said nozzles.
10. A device according to claim 1, wherein the transducers are
defined in a plurality of members supported on a substrate.
11. A device according to claim 1 wherein the transducers are
flexural.
12. A device according to claim 1, wherein the transducers are
formed in regions of a material layer having a thickness reduced
from that of the remainder of the material layer.
13. A device according to claim 1, wherein the transducers are
formed in a first material layer and separated from one another by
a sealing layer, and the nozzles are formed in the sealing
layer.
14. A device according to claim 13, wherein each nozzle is disposed
between the ends of a pair of opposed transducers.
15. A device according to claim 13, wherein each nozzle is disposed
at the end of a respective transducer.
16. A device according to claim 1, wherein the transducers comprise
beams, each beam having a free end which is supported by a
stiffening layer of material.
17. A device according to claim 1, wherein the transducers comprise
beams, the side of each of which are tapered towards one another,
the respective nozzles being located in the narrower portions of
the respective beams.
18. A device according to claim 1, wherein a plurality of signal
terminals are provided corresponding to respective transducers.
19. A device according to claim 18, further including an electronic
drive coupled to the terminals and hence to the transducers, and
arranged to provide drive signals independently to respective
transducer terminals, whereby production of droplets from the
nozzles is achieved selectively by corresponding selective
generation of drive signals.
20. A device according to claim 1, including further transducers
with which no nozzle is associated, whereby actuation of the
further transducers can be employed solely to reduce crosstalk
between adjacent nozzles.
Description
The present invention relates to liquid projection apparatus in the
form of what is known as a `face-shooter` array.
In the ink-jet art there are many liquid projection devices that
utilize the acoustic resonance of capillary channels or chambers,
hereinafter collectively termed `cells`, associated with nozzles to
provide a pressure wave to cause liquid to be expelled from those
nozzles. These technologies are limited in their maximum frequency
of operation by the liquid acoustic resonant frequency of these
cells. In addition to this, the cells act as a restriction to flow
causing pressure to be developed within the cells which effects
drop ejection. Flow through the cells is therefore limited by the
refill rate, causing a further upper limit to the operational
frequencies of such devices. Moreover, the cells act as a trap for
air bubbles and contaminant particles that severely disrupt
operation and which are problematic to remove. Structures employing
cells are also thereby restricted to handling liquids of particular
rheology, high purity and stability. For example, unstable
suspensions that are used to form white, gold and silver inks
cannot be applied reliably with devices employing cells.
Further technologies described in the art provide excitational
members in the bulk liquid in close proximity to the rear of a
separate nozzle-plate. This configuration has the advantage of
allowing bubbles to escape, but the method is intrinsically
inefficient in use of energy and prone to crosstalk.
A further difficulty associated with printheads known in the art is
that their construction is based on sub-assembly onto a three
dimensional structure rather than onto a substantially two
dimensional workpiece. This has the consequences of increasing the
variability of the product and decreasing the manufacturing
yield.
In the ink-jet industry, there is an unfulfilled requirement for
printheads that can be fabricated with a sufficient number of
ejection sites on a single printhead to constitute a page-wide
array. The problem of producing such a printhead is the requirement
that the fabrication processes must provide a structure with a high
degree of consistency between thousands of ejection sites.
Predominantly in the prior art, for example EP0728583A2,
constructions are taught wherein there is a requirement to locate a
number of components in three dimensions to form a linear drop on
demand ink-jet printhead. This construction does not achieve the
integration of the transducer means into a substantially planar
form. It is the belief of the authors of the current invention that
this lack of integration is the prime limitation on the width of
the array that can be constructed by methods in the prior art.
The same construction difficulty arises in WO-A-94/22592 wherein
excitation means are bonded to, but not integrated with, a material
layer in which nozzles are formed. This prior art also requires an
extended structure behind the nozzle-plate to provide the acoustic
energy to enable drop production. The fabrication of this prior art
must also proceed by fabrication in three dimensions, again
introducing problems of component alignment.
As stated above, the inventors believe the solution to this problem
is to understand that the extended construction can be achieved by
integrating the transduction means to the surface. In practice,
achieving this is not a trivial problem to overcome.
For example, if the structures disclosed in WO-A-94/22592 are
reduced in height to achieve the substantially planar condition, a
contradiction results. Firstly, the motional action of the PZT
(piezoelectric lead zirconate titanate ceramic) as configured,
reduces markedly with PZT height reduction. Secondly, the structure
requires the surface to flex, but the PZT must remain rigidly
bonded to a further surface. The structure cannot be reduced to a
layer.
A second approach is to apply an annular ring geometry, such as is
described in our EP-A-0615470, to form a surface array of such
devices. If the flexural rings are set out and connected in an
array there is first a problem of scale; the separation of nozzles
on the array will be determined by the minimum achievable PZT ring
outer diameter that will produce droplets at an acceptable drive
voltage. This will be too large to form a high resolution linear
array (such as 150 nozzles per inch, as is required, for example,
in many printing applications). An attempt to apply the vibration
of a surface (bimorph) flexural ring is disclosed in JP 09-226111.
However in this case the rings being bonded to, or formed in, a
material layer are unavoidably coupled around the outer
circumference to the material layer as a whole, inducing
undesirable crosstalk between rings.
An alternative form of excitation, from a similar physical
structure to JP 09-226111, is shown in JP 10-58672. In this patent
application, the radial vibration of a surface ring is apparent.
Again, however, the rings are unavoidably coupled around their
outer circumference to the material layer as a whole, inducing
undesirable crosstalk.
A subsequent patent application, JP 10-58673, also discloses an
annular ring geometry employed to produce a surface meniscus
resonant wave. The inventors of JP 10-58673 seek to improve fluid
coupling by introducing a further structure at a defined depth of
ink beneath the nozzle, forming a flow constriction which
effectively defines a resonant cell structure, and thereby the
substantially planar condition of construction is lost.
In the prior constructions taught in JP 09-226111, JP 10-58672, JP
10-58673, the droplet formed is small compared to the `nozzle`
opening of the substrate. The relatively large nozzle is
correspondingly more sensitive to undesirable wetting of the front
face of the printhead than the constructions in which an issuing
jet fills the nozzle to produce a droplet of similar size, for
example under conditions of ink having low surface tension or
physical shock to the printhead. This sensitivity arises from the
lower pressure differences that the relatively larger diameter
meniscus within the larger `nozzle` can sustain.
According to the present invention there is provided a device for
projecting liquid as jets or droplets from multiple nozzles, the
device comprising;
a plurality of transducers oriented substantially parallel to one
another and each having an inner face and an outer face opposite
said inner face, the transducers being arranged in a substantially
planar array;
a plurality of nozzles, each nozzle being associated with a
respective transducer which is excitable to cause movement of the
associated nozzle in a direction substantially aligned with the
nozzle axis, to project liquid therefrom;
liquid supply means for supplying a liquid to said inner face of
said nozzles;
means for selectively exciting transducers as required, thereby to
project liquid as jets or droplets from the respective outer face
by movement of the liquid through the nozzle in response to the
movement of the nozzle.
Thus, in such a device, the transducers are all aligned in the same
direction and, where the transducers are rectilinear, they all have
a major axis parallel with that of the other transducers. Even
where the transducers are not rectilinear, as long as they are
congruent, they will have at least one edge in parallel with the
same edges of other transducers in the array.
By the term "transducer" is meant a local region of the liquid
projection device that can be stimulated into motion by an
associated individually-addressable excitation means. By the term
"substantially planar" it is meant that the height of the
components is small with respect to the lateral extent of the array
of individual components.
The inventors believe the key to achieving a page-wide array is
making the array in layers which may be aligned optically using
surface processing techniques.
The transducer components may be formed in one piece comprising,
for example, a piezoelectric or similar excitation means. The
transducers may also be formed as a composite component in which an
excitation means is, for example, bonded to or formed integrally
with one or more other material bodies which may, for example,
provide a mounting support or substrate for the excitation
means.
It is not necessary that all transducers have nozzles associated
with them. However, for those transducers that do have associated
nozzles, the nozzles may pass through the exciting means or through
the material body (or bodies) that, together with the exciting
means forms the composite transducer or through both the exciting
means and that material body (or bodies). In each case the
transducer surfaces that each nozzle intersects in passing
therethrough define the inner and outer faces of that transducer.
Correspondingly, in this specification, implementations of the
invention in which a nozzle (or nozzles) is formed in a separate
component from that to which excitation is directly applied are
together considered to comprise the transducer.
Preferably the excitation means, and associated material body (or
bodies) if used, that form the transducer are in the form of
layers. Forming the transducers of the projection device from layer
components in this way allows accurate registration of their
component parts to be more easily and reliably achieved in assembly
of the liquid projection device than is achieved with the
three-dimensional constructions prevalent in the prior art.
Distinct transducer regions may be formed within the material layer
by selective thinning of the layer, allowing the respective region
to move with reduced constraint from the remainder of the material
layer and thereby enhancing transducer operation. A further
reduction in constraint can be achieved by slitting right through
the material layer to form slits around each transducer region. The
regions may thus be in the form of beams formed by slits within or
through the material layer, and each of the slits may be sealed.
Furthermore, the slits may be arranged in the form of a comb or in
the form of two interdigitated combs.
In the prior art slits were included simply to enable bending to
take place and the have to be thin otherwise they will leak--so
they are a mixed blessing. We have turned the problem to our
advantage by using the slits not only as a means of getting the
bending, but also as a method of suppressing crosstalk--so the
slits become isolation means decoupling the transducers. The
isolation can be improved by filling the slits with a compliant
medium, and this overcomes the leakage problem. The slits may be of
comparable width to the transducer as the compliant medium allows
the separation to be chosen to get the isolation needed.
Decoupling is thus achieved by separating the transducers by
substantially parallel gaps within or through the surface, with the
principal constraint and contact of the transducers to the bulk of
the material layer adjacent to the distal ends of those gaps, and
preferably with the greatest amplitude of transducer motion
proximal to a nozzle opening (preferably located distant from those
distal ends) which ejects the liquid (typically ink). Further, the
physical separation allows a second material to be used to fill the
space between the transducers. If this is chosen to be a compliant
rather than a stiff medium, excellent decoupling can be achieved.
Alternatively a compliant sealing layer may be used to seal the
space, again maintaining excellent decoupling. The use of slits to
divide flexural nozzle-plates is disclosed in WO-A-94/22592, but a
planar array of transducers bearing nozzles is not taught, and the
width of the slits is restricted by the tendency of the liquid to
seep or be pumped to the outer surface during filling and drop
firing respectively. In the present invention, nozzle-plate motion
is induced by flexural, rather than extensional, elongation of
rigid rods, as in WO-A-94/22592 Therefore, in the present
invention, the mechanical properties, e.g. stiffness, of the
nozzle-bearing layer are comparable to those of the `excitation
means` layer, helping to maintain co-planarity of adjacent
nozzle-bearing transducers. This helps both to prevent liquid
egress through unsealed slits and, in the case of sealed slits,
helps retain motional excitation to produce droplets with low or
acceptable levels of crosstalk carried by the sealing means. The
use of sealing means for liquid sealing without introducing
substantial crosstalk is therefore one aspect of the current
invention.
The layer-surface may be utilised as part of the transducer means,
selectively decoupled from the extended material layer to suppress
crosstalk and configuring the excitation means within this
transducer to function jointly in the bending mode. Further, this
new layer-surface approach allows the use of nozzles of dimension
smaller than the diameter of ejected droplets (and with good
resistance to blocking in the case of suspension liquids such as
pigmented inks), so avoiding the sensitivity to `wetting-out` shown
by the prior art devices.
In one construction arising from the invention the transducers may
comprise three material layers, each optimized for their function,
for example; a first layer of piezoelectric material providing the
excitation means and which is mounted on a second support layer of
(for example) stainless steel sheet that cooperates with the
piezoelectric layer to provide flexure and which in turn carries on
its opposite face a third thin polymer layer, in which the
liquid-ejection nozzles are formed. Alternatively, such functions
may be combined into two or even one layer.
For those transducers bearing nozzles, we term the local vicinities
of the nozzles of those transducers as the `nozzle regions`. In
use, liquid present at a nozzle region at the inner face of a
transducer passes through the nozzle to be projected as a jet or
droplet (or plurality of jets or droplets) when the transducer is
excited such that at least the nozzle region moves (with
appropriate amplitude and response time) in a direction
substantially aligned with the nozzle axis. Most conveniently both
the nozzle axis and the motion of the nozzle region is in a
direction substantially parallel to the surface normal of the
nozzle-bearing region of the transducer.
The nozzles within the nozzle bearing regions are arranged in an
array within the device, which array may be one-dimensional such as
a row or a line of nozzles, or may be two-dimensional such as
multiple rows or lines preferentially arranged parallel to each
other. Such a nozzle array ensures that there is an array of, at
least, nozzle-bearing transducers. In addition, within that array
may be additional transducers without nozzles (for example
interspersed with the nozzle-bearing transducers). These additional
transducers can be helpful in suppressing residual
crosstalk-induced layer resonances and layer edge effects.
In preferred embodiments of the invention at least those
transducers bearing the nozzle-bearing transducers are individually
addressable. It is usually desirable that the motion of one nozzle
bearing transducer (excited to eject liquid from its corresponding
nozzle(s)) does not cause comparable motion of other nozzle-bearing
transducers or substantial pressure fluctuations in liquid adjacent
nozzle-bearing regions of other transducers. In this way, not only
are the nozzle-bearing transducers individually addressable, but in
addition individual control of liquid projection may be obtained
from each nozzle-bearing transducer and, where each such transducer
includes only one nozzle, individual control of ejection from each
nozzle is obtained. We refer to this generally as reducing
`crosstalk` between nozzles (and/or nozzle-bearing
transducers).
The array of transducers referred to above, and/or any of their
constituent parts (such as excitation means, support body for
excitation means and/or nozzle-bearing body) may be integrally
formed with one another or else may be individually formed. If
integrally formed then, to reduce crosstalk mediated through the
solid elements of the device (`mechanical crosstalk`), it is
generally desirable to separate or partially separate them by gaps,
typically in the form of slits. The gaps may extend through one,
several or all component layers of the transducers (for example
through just the exiting means and its support body but not through
a thin polymer nozzle-bearing layer).
In the case that the slits or gaps extend through all those
components that otherwise provide a slit between inner and outer
face of the transducer it is generally advantageous to seal those
slits or gaps against liquid egress or evaporation. This may be
done, for example, by incorporating into the slit a soft elastic
material body that is poor at transmitting transducer motion
parallel to the slit, such as Latex Solder Resist as supplied by
RS, part number 561549. Alternatively a further material layer (or
layers) (which can be considered, to the extent that it contributes
to transducer operation as a further transducer component; and
which can be considered, to the extent that it influences only
overall device performance, as a common component for all the
transducers that it seals in this way) can be applied across the
slits thereby to seal them. This further material layer may be
formed of, for example, 25 micron polyimide sheet such as
Upilex.
Therefore, in favoured implementations of the present invention,
there is provided a material layer or layers bearing nozzles and
which are excited into motion with respect to bulk liquid brought
to them. This action induces pressure excursions in the bulk liquid
in nozzle regions of transducers. Each transducer bearing a nozzle
is individually excited into motion (`individually addressable`),
and the invention allows simple construction of such individually
addressable multi-nozzle droplet dispensing devices.
This invention aids the reduction of mechanical crosstalk between
nozzles, thereby aiding individual control of liquid ejection from
each nozzle as well as providing individually addressable
transducers.
Devices according to the invention may with advantage provoke both
positive and negative excursions of such liquid pressure at least
in nozzle-bearing regions to project liquid from the said nozzle.
The `motional nozzle` method does not rely upon low compressibility
of the liquid or upon stiff cells and so contrasts with
conventional ink-jet droplet dispensation apparatus wherein
pressure is generated compressively within the cells.
The current invention provides `direct` excitation of the nozzle
region (`direct` in this sense meaning that the excitation is not
primarily transmitted to the nozzle region by using the liquid as
the transmission medium. Rather, it is primarily transmitted via
the solid material components of which each respective transducer
is formed. In this way a device according to the invention causes
the greatest pressure excursions to be produced in the
nozzle-bearing region immediately behind the nozzle, this
advantageously thereby reduces `liquid crosstalk` mediated by the
liquid. By `liquid crosstalk` we mean energy transfer via the
liquid from the nozzle region of one transducer to the nozzle
region of another transducer (otherwise, potentially making an
undesirable contribution to liquid ejection also from that other
nozzle).
A unique advantage of devices according to the present invention
over devices in the prior art is that the individual addressability
and commonality of substrate of the transducers advantageously
permit active cancellation of residual crosstalk signals from one
local region by the partial or phased (or both) activation of
neighbouring transducers. This results in the ability to operate
next nearest neighbour local regions synchronously, using the
intervening transducers (which may have no nozzles) to actively
damp crosstalk.
The integration of the excitation and nozzle means and the motional
excitation mechanisms also reduces or eliminates the need for
separate liquid cells for each nozzle. In turn, this suppresses the
sensitivity of jet or droplet projection to bubbles in the liquid
which, in cell-based designs, can become trapped in those cells to
provide continuing perturbation of jet and/or droplet
projection.
The current invention also allows the high accuracy components to
be concentrated in, at most, a few, sheet-like layers. This allows
ease of fabrication since the piece parts are assembled onto a
single plane.
The liquid projection apparatus described herein is believed by the
inventors to be unique in the ability to deposit, white, gold and
silver inks, and other inks of large pigment size and unstable
dispersion characteristics, due to the unique ultrasonic action of
the transducer means.
In addition, the action of the motional drive of at least the
nozzle regions allows the device to perform an ultrasonic cleaning
action of at least those regions of the transducers, including the
inner and outer faces of those regions and of the nozzles
themselves. This allows maintenance with reduced need for purging
and wiping of the face of the device.
Advantageously, the excitation of the transducer can substantially
localise the stimulation pressure to that liquid directly in
contact with the nozzle region. This can be achieved by, for
example, making the nozzle region less stiff to bending motion than
the rest of the transducer, so that the greatest motional response
(and thereby the greatest stimulation pressure) occurs in the
nozzle region itself.
This means that in the new liquid projection device a very sharp
resonance is not necessary and the liquid projection is thus
further less sensitive to factors such as the consistency of the
liquid, the presence of air bubbles in the liquid and manufacturing
tolerances of the device, which generally have a very serious
effect on the performance and cost of a conventional liquid
projection device. The new liquid projection device is therefore
potentially cheaper and more reliable in operation than the prior
art and does not require such complicated liquid conditioning
apparatus.
Advantageously, the thickness of each transducer in the motional
direction satisfies the inequality: ##EQU1##
where t, is the thickness of the rth layer of material in the
transducer and c, is the speed in that layer, at the operating
frequency f, of either compressional or shear waves propagating the
layer in the direction of its thickness.
Other excitation means than piezoelectric elements suitable for use
with the invention are electrostrictively, magnetostrictively and
electrostatically deflected electromechanical elements.
In one example embodiment, piezoelectric elements are used as the
excitation means, exciting motion of material layer(s) bearing
nozzles responsive to an electric field applied to those elements.
The elements are in the form of thin layers of piezoelectric
material having electrodes on opposing faces. When pre-formed as
fired elements, one such face of each piezoelectric element is
mechanically bonded to a part of the nozzle-bearing material layer.
When a refractory nozzle-bearing layer material (such as ceramic)
is used, the piezoelectric elements may alternatively be deposited
as a thick film (for example by screen printing) and fired in situ
to form the excitation means. In both cases the piezoelectric layer
is arranged to expand or contract on application of a voltage to
it. Thus each element in combination with that area of the
nozzle-bearing material layer to which it is bonded cooperatively
form a transducer in the form of a flexural member. A nozzle formed
either in this bonded area or in a nearby area of the
nozzle-bearing layer thereby completes formation of a
nozzle-bearing transducer. Both nozzle-bearing transducers and
non-nozzle-bearing transducers can thereby be excited into bending
motion substantially perpendicular to the electroded surfaces of
the piezoelectric element and of the transducer as a whole. This
provides, as a first advantage, motional excitations of the
transducer and the nozzle-region within it, in a simple and
efficient manner.
A second advantage resulting from this embodiment is that the
exciting part of such a nozzle-bearing transducer structure (in
this case the piezoelectric element and the region of the
nozzle-bearing material layer to which it is bonded) can have a
much lower acoustic impedance than in a conventional liquid
projection device, the acoustic impedance of this exciting part
being comparable to that of the nozzle region (and the same
impedance if the nozzle lies within the exciting part). These
circumstances mean that the amount of excitational energy stored in
such a transducer is smaller than that stored in conventional
devices and that a larger amount of energy can be transferred
during excitation in either direction between the exciting part and
the nozzle region. This makes it possible to control the excitation
of the nozzle region directly by feeding appropriate drive signals
to the excitation means, so allowing unwanted motions to be
actively suppressed.
The isolation of one transducer from another (reduction of
crosstalk) may be improved by fabricating transducers separated by
gaps, or by locally removing material to form gaps within layers
that are common to other transducers, particularly other adjacent
transducers, thereby reducing the mechanical coupling between them.
This may be achieved, for example, by grinding or by laser cutting,
the latter advantageously allowing the fabrication of narrow slots
of approximately 5 microns width, producing well-defined slits,
free of blade "run-out".
The transducer may include an additional substrate (preferably, but
not necessarily, in the form of a layer) on which said excitation
means is mounted, said substrate having an aperture, and a flexible
membrane mounted on said substrate and covering said aperture,
wherein said nozzles extend through the region of the flexible
membrane that covers said aperture. In such a construction
employing a separate substrate and flexible membrane, the flexible
membrane may be bonded to the substrate which may be formed of, for
example, stainless steel.
In many applications of the present invention it is desirable to
arrange the multiple nozzles in an array (or series of arrays) each
being individually addressable by the excitation means of its
respective transducer. These arrays of nozzles may be formed to
define a common outer surface which surface may conveniently be
coincident with the outer face of a common nozzle-bearing layer. In
this case, preferably, the shape and location of the, or each,
excitation means and transducer is arranged to avoid generating
travelling waves which transfer energy between transducers from one
nozzle to another, so mechanical crosstalk is minimized. This may
be achieved for example by forming slits (described above) in the
nozzle-bearing layer and/or in any auxiliary material layers
employed.
Further refinement may be achieved by providing sensing means
separate to or integral with the excitation means of nozzle-bearing
transducers and by using feedback from the sensing means to nullify
the background noise. Similarly, transducers without nozzles may
alternatively or additionally, be excited to provide motional or
pressure damping or cancellation in the nozzle regions of
nozzle-bearing transducers. For this function such transducers
without nozzles may beneficially be interspersed between
nozzle-bearing transducers to form an alternating array. Indeed,
benefit may be found even by the provision of simple `passive
elements` between nozzle-bearing transducers, these `passive
elements` typically comprising nozzle-less transducers that also
have no excitation means or no drive provided to any excitation
means that they do possess.
The nozzle-bearing layer may be mounted on a manifold having a
cavity for supplying ink to at least the nozzle regions. The
manifold may include excitation damping materials or otherwise be
designed to inhibit resonance and, by extending across all or
several nozzles can avoid the `cell` construction of most
conventional ink-jet printheads with their associated sensitivity
to air bubbles and solids deposition.
The liquid projection device may also be formed as a piecewise
fabrication of preformed components. This advantageously allows the
choice of boundary conditions for the transducers, the pre-testing
of component parts and the employment of further layers for nozzle
regions and sealing structures between transducers.
Preferably, the device includes an electronic drive coupled to the
terminals and hence to the transducers, and arranged to provide
drive signals independently to respective transducer terminals,
whereby production of droplets from the nozzles is achieved
selectively by corresponding selective generation of drive
signals.
A number of examples of devices constructed in accordance with the
present invention will now be described with reference to the
accompanying drawings, which include:
FIG. 1a a cross-section of a device illustrating, in simplified
form, the principle of operation during a push stroke;
FIG. 1b a cross-section of a device illustrating, in simplified
form, the principle of operation during a pull stroke;
FIG. 2 a plan view of a first device;
FIG. 3 Finite Element modelling result for frequency response
function of the first device;
FIG. 4 a graph of an experimental frequency response function of
the first device;
FIGS. 5a, b, c plan views of three further examples;
FIGS. 6a, b plan views of two further examples of constructions
using piecewise assembly methods;
FIG. 7 a plan view of a device having an interdigitated cantilever
beam construction;
FIG. 8 a plan view of a further device having an interdigitated
cantilever beam construction;
FIG. 9 a partial plan view of a device constructed using multiple
material layers;
FIG. 10 a cross-section of the device of FIG. 9;
FIG. 11 an isometric view of a further device embodying selective
thinning of the transducer beams;
FIG. 12 an isometric view of a PZT fabrication for use in a device
according to the invention
FIG. 13 a cross-section of a device incorporating the PZT
fabrication of FIG. 11
FIG. 14 a plane view of a construction having tapered beams
FIG. 15 a partial cross-section of a device having a slotted PZT
construction;
FIG. 16 Finite Element modelling results with variable thickness of
sealing layer;
FIG. 17 a plan view of a still further example;
FIG. 18 a plan view of a layered construction with additional
support at the ends of PZT elements;
FIG. 19 a partial cross-section of a further example;
FIG. 20 a plan view of a device having a two-dimensional array of
nozzles;
FIG. 20a a plan view of a magnified portion of the device of FIG.
20;
FIG. 21 a schematic of a device configuration; and
FIG. 22 a schematic graph of a suitable drive waveform.
FIG. 1 a shows nozzle-bearing members 1 formed in thin material
layers, so providing extremely short inertial and viscous effective
lengths for flow of liquid 2 through nozzle 8 in direction shown at
98 to form the emergent liquid 3 responsive to motion shown at 4
that induces positive pressure excursions in liquid 2. FIG. 1b
shows flow of liquid 2 in direction shown at 99 responsive to the
motion shown at 5 that causes negative pressure excursions in
liquid 2, in turn causing the emergent liquid 3 to form emergent
droplets shown at 100. This, together with the ability of devices
according to this invention to provide pressure excursions of time
duration in the region of one micro-second to one milli-second,
advantageously allows liquid projection at very high
frequencies.
One example embodiment, which has been reduced to practice, of a
single transducer of the overall array device, is shown in plan
view in FIG. 2. This illustrates a transducer 9 incorporating a
`beam` 6, with, for example, two piezoelectric elements 7 formed of
PZT per nozzle 8. Nozzle 8 penetrates through material layer 100.
This construction can provide a nozzle hole 8 mounted precisely at
the motional antinode of the transducer, giving a symmetric
pressure distribution in the sub-region of the nozzle hole. The
transducer 9 is distinctly formed, in this case, by the
introduction of slits 10 into material layer 100.
In this example as an operating liquid projection device, material
layer 100 is electroformed nickel of 100 micron thickness and
bearing a nozzle of exit diameter 25 microns. The slits 10 were
formed by electroforming and are of width 20 microns; slit length
is 9 mm, and the distance between the slits 10 is 1 mm. The
piezoelectric components 7 each have width 0.8 mm, length 1.5 mm,
thickness 200 microns, and are formed of piezoelectric ceramic P5
sourced from Ceramtec of Lauf, Germany providing high piezoelectric
constants and mechanical strength. The electrode material applied
to said piezoelectric components 7 was sputtered
nickel-cobalt-gold, of thickness in the region 2-5 microns. This
allowed cutting action with negligible damage to the PZT material
or slitting saw. This also allowed electrical connections to be
made to the transducers using 30 micron diameter aluminium wire,
using an ultrasonic wire-bonder. The piezoelectric components 7
were bonded to nozzle-plate 100 using adhesive Araldite 2019
supplied by Ciba-Geigy, UK.
Continuously stimulating excitation of the beam motion alternately
in each direction at the resonant frequency allows such a device to
eject a continuous stream of droplets. The device above produced a
continuous droplet stream when excited by an alternating
square-wave voltage of 120 Volts peak to peak, at a frequency of
95.8 kHz.
By stimulating excitation with only one or a discrete number of
such cycles allows the device to eject droplets `on demand` i.e.
responsive to that short droplet-projection pulse or pulse train,
and ceasing after that pulse train ceases. The device described
above was operated with a drive voltage of 150V peak to peak and
with a base frequency 97.3 kHz. This device yielded a maximum
`on-demand` ejection frequency of 4 kHz. With other devices of this
general form, on-demand frequencies of up to 10 kHz have been
observed with a drive voltage 40V peak-to-peak.
Using water-based ink, at a supply bias pressure from 0 to -30
mbar, the device was demonstrated operating in drop on demand mode.
This liquid projection apparatus whose fabrication was described
above was mounted onto a manifold to provide liquid supply means
and in proximity to printing media to form a system suitable for
ink-jet printing. It was found experimentally that no sealant was
needed in order to prevent egress of fluid from the slits, although
for long-term reliability a sealing layer would be necessary.
The motional response peak for the device of FIG. 2 as a function
of frequency, as predicted by finite element modelling, is shown in
FIG. 3, and is typically broad. The frequency scale runs from 80
kHz to 100 kHz, showing a predicted maximum amplitude at frequency
87 kHz. It also shows the absence of unwanted vibrational modes
near to the desired operating frequency.
FIG. 4 shows the result of experimental measurement of the
electrical impedance phase using a HP 4194 impedance spectrometer.
The frequency sweep runs from 50 kHz to 150 kHz, and shows that the
only resonance in this range is the broad peak centred at 99.5
kHz.
In alternative constructions for the example of FIG. 2, unimorph
(single layer) and bimorph (double layer) geometries may both be
employed for the excitation means shown at 7. The thickness of the
region of material layer material 100 near the ends of the slots is
chosen to control the resonant frequency of the device.
Being substantially isolated by slits 10, arrays of such
transducers allow substantially independent control of drop
ejection from an array liquid projection device such as an ink-jet
printhead.
FIGS. 5a, 5b and 5c illustrate optional constructions wherein
multiple nozzle-bearing transducers 9 are formed within the
material layer 11, their lateral extent being defined by slits 12.
Each such transducer bears a nozzle 13 through layer 11. FIGS. 5a,
5b and 5c differ in that they illustrate a variety of permutations
of excitation means configuration 14, as shown.
The device may also be constructed from an assembly of pre-formed
transducers (nozzle-bearing or otherwise) in the form of one or
more linear arrays with a choice of excitation mode and acoustic
boundary conditions available, including fixed-free (cantilever) or
fixed-hinged, or fixed-fixed boundary conditions. Separate
transducers may be assembled with fixed-free, fixed-fixed,
hinged-fixed, or hinged-hinged boundary conditions, as appropriate
to achieve the desired resonant conditions. Here, the terms
`hinged` and `pivoted` are treated as synonymous, as are the terms
`clamped` and `fixed`, as is customary in acoustic theory.
Such an assembly of pre-formed regions is shown in FIGS. 6a and 6b,
wherein the material layer 15 having an aperture (or apertures) 10,
forms a base for the attachment of transducers 16 (including
excitation means 20) which may themselves include pre-formed
apertures 17, or be left blank 18, so that these blank members may
be used as active crosstalk compensation means, as shown with
reference to excitation means 19. To apply the illustrated means as
liquid projection apparatus, the apertures 17 and interstitial
regions between transducers 16 (and between transducers 16 and
plate 15) are themselves sealed by a further layer in which nozzles
are formed in regions corresponding to apertures 17, or may
themselves be formed as nozzles.
FIG. 7 shows an assembly constructed out of a material layer 110 in
which two sets of cantilever beams are formed as interdigitated
combs 22 and 23, the teeth of the combs providing flexural
nozzle-bearing transducers. The combs are formed in a material
layer 110 bonded to a sealing layer 101. In embodiments where the
material layer 110 is formed of excitation material such as
piezoelectric material, local areas 102, 103 of that material can
be electroded and activated in those regions by the use of
pattern-track 104 and pad connections 105 formed on the material
layer itself. Nozzle means 106 can then be formed through the
flexural transducer 108 or sealing layer 101. The pad connections
can be arranged to accept the array contacts from driver integrated
circuits advantageously eliminating the need for high density
electrical connections to the array of flexural members.
FIG. 8 illustrates a variation on the embodiment in FIG. 7, wherein
the nozzle-region 107, as indicated by the dashed circle, is
associated with, but not formed through the transducer (in this
case a flexural transducer) shown by dashed line 109. This
variation allows the flexible sealing layer 101 to incorporate the
nozzles 75, which is advantageous when, for example, the formation
of the nozzle through the sealing layer is simpler and more
accurate than through the transducer material itself. In this case
also, the material layer 110 bears only the cantilever beams 22,
23; the excitation means 102, pattern track interconnection 104,
and pad connections 105 are formed on the nozzle-bearing and
sealing layer 101.
In FIGS. 7 and 8, the sealing layer 101 may be formed of Upilex of,
for example 25 micron thickness.
FIGS. 9 and 10 illustrate in schematic plan and section at AA
respectively, the construction of flexural transducers, such as one
indicated at 24 to be used within the overall array device wherein
pre-formed slits 10 and aperture 25 in material sheet 100 and
excitation means 29 are overlaid with a nozzle-bearing layer 26.
This construction thereby allows separate nozzle fabrication and
slit sealing means (shown as material layer 100 in FIG. 9), with
the nozzle 8 advantageously located at the antinode of the motion
of the flexural means.
The nozzle-bearing layer 26 overlaid on material sheet 100 which
includes receiving pockets 28 for the excitation means 29, the
structure then being bonded to a support means 30, that may also be
used as part of a liquid supply means.
Nozzles formed in the construction of the liquid projection device
may have cylindrical form or other form with tapered cross-section.
The tapering of the nozzles may result in the opening at the inner
face being smaller than the outer face which is a form well known
in the art for ink-jet printing. Alternatively the opening at the
outer face may be formed to be smaller than the opening at the
inner face, providing different operating regimes as described in
relation to aerosol applications in our granted patent EP-B-0732975
and co-pending patent application GB9903433.2.
The nozzle-bearing layer 26 or support layer 30 may advantageously
be made of stainless steel. Chemical etching or laser ablation of
this material allows a simple method for the fabrication of
stress-free substrates with small and reasonably well-characterized
nozzle holes.
In a further embodiment of a transducer suitable for use within the
current invention, a construction is shown in FIG. 11 wherein the
material layer 31 is formed from a plate of thickness large enough
to give good coupling between the motion of the PZT and the
flexural motion of the plate. The locally thinned regions 32 of
that layer 31 give increased amplitude motion at the nozzle for a
given voltage. Such an embodiment could be fabricated, for example,
by electroforming.
The liquid supply pressure may be controlled or the volume of ink
delivered restricted enabling the device to be stored empty in a
capping and maintenance station. This advantageously reduces the
effects of nozzle-blocking due to evaporation from the liquid
menisci in the nozzles while the device is not in use.
Additional cleaning in the maintenance station may advantageously
be achieved by ultrasonic vibration of the device using the
excitation means at the normal operational frequency or at a
separate frequency chosen to cause cleaning of the material layer.
The vibration may alternatively be supplied by a separate
excitation means which may be mounted on the maintenance station or
supplied by separate excitation means used in operation for active
damping located on or in the proximity of the material layer.
In the above embodiments the nozzles and slits may be alternatively
formed in nickel by electroforming, the PZT may then be bonded onto
the nickel. Alternatively, only the nozzle holes may be formed by
the electroforming step, in which case, the slots may be laser cut
though the nickel. In both cases a laser or grinding saw can be
used to cut slots through the PZT. Use of electroformed nickel
advantageously allows patterned resist techniques to be applied for
the lithographic definition of slits and nozzles as a single or two
stage process.
In designs wherein the nozzle-bearing layer is formed separately
from a slit-sealing layer, the slit-sealing is preferably provided
as a compliant membrane (or membranes) such as 25 micron thick
Kapton, which seals the slits but leaves the nozzles open. This
advantageously ensures that liquid is not ejected from the slits
and prevents evaporation of liquid from the slits that could
otherwise inhibit motion of the nozzle regions and/or their
associated transducers.
Preferred fabrication routes are now discussed which advantageously
provide good nozzle hole quality and narrow pitch between nozzles.
Laser machining techniques, particularly excimer lasers and
frequency-tripled pulse YAG lasers, can give good quality slits and
nozzle hole quality in a range of materials. In practice we find
that an excimer laser, in particular a Lambda Physik model Minex
30796 producing 300 mW power at 248 nm with 40 Hz repetition rate
is well suited to the fabrication of nozzles and slits made in PZT.
Good quality nozzle holes in PZT, with diameter 25 microns are
machined in 10 seconds. Slits in PZT can be machined by scanning
the device through the laser beam. The material ablation rate was
found to be approximately 20 microns/sec or equivalently 0.5
microns/pulse. In large scale production, nozzles and slits of
approximately one transducer per second could be fabricated using
this fabrication route, with only a small contribution to the cost
per nozzle of the print-head.
In a yet further embodiment, the structure may be achieved by the
application of anisotropically etched silicon substrates, which
advantageously provide; large nozzle taper angle (the 54.7 degree
angle between the silicon 111 and 100 planes is convenient exposed
by wet chemical etching with KOH solutions, this is well known in
the art), giving ratio of 2:1 or higher between hole minimum and
maximum diameters; improved channel-to-channel consistency;
fabrication techniques commonly used in mass-production in the
semiconductor industry.
In forming an array of such devices, individual transducers may be
formed using from a monolithic or multilayer slab of excitation
material such as a piezoelectric ceramic (PZT) layer. As shown in
FIG. 12, a central groove 35 is first cut in a monolithic layer 36
of such material. This defines a common inner edge of all
transducers 37 in the transducers array, the outer edge being
formed by the periphery of the monolithic slab 36. The individual
transducer elements 37 are then defined by cross-cutting the
structure. This `totem pole` structure is then inverted and bonded
onto a further material layer so forming an array of flexural
transducers. In the case where that material layer is a
nozzle-bearing layer, this procedure advantageously aligns and
places a number of transducers relative to nozzles in a single
step. After bonding, the transducers are then separated from one
another by one or more dicing cuts that remove the remaining
material of central groove region.
A cross-sectional view of a transducer fabricated in this manner is
shown by dashed line 94 of FIG. 13, encompassing PZT elements 39
bonded to a nozzle-bearing material layer 42. This structure is
then prepared for electrical interconnect by the insertion of
spacer material layer 38 (that preferably is of a material with a
high thermal conductivity) behind the PZT elements 39. An
interconnect and protective layer 40 with pattern tracked
electrodes 41 is then bonded over 38 and 39 thereby providing
individual addressing means to the PZT elements 39, the ground
plane connection being provided by the material layer 42 (either
directly if layer 42 is conducting, or by means of an electrode or
electrodes pre-formed onto layer 42 if that layer is chosen of
non-conducting material). A fillet 43 is then applied to the edge
of the transducer elements, sealing them from contact with fluid
protecting them from chemical or electrical attack of the
electrodes and piezoelectric elements. That assembly may be bonded
to an ink manifold 30 either way up, by choice. The slits may be
sealed using the fillet material or by providing an additional
sealing layer 73.
Alternatively, the PZT elements, interconnect and spacer layers and
glue fillet, lie on the fluid side of the device, with manifold 30.
This configuration also has the advantages of protecting the PZT
from mechanical damage in use and provides a planar top surface for
ease of device maintenance in capping, purging and cleaning. In
either case, the fluid can act as coolant to the excitation
components.
An alternative in the construction method of devices as shown in
FIG. 13 is provided by using the interconnect and protective layer
40 as the locator for the PZT elements 37 which are first mounted
to the layer 40 before being joined to the nozzle-bearing layer 42.
The PZT may be formed into excitation elements by methods described
earlier or alternatively be formed individually and put in position
by pick and place machinery. Either of layers 42 or 40 can also be
fashioned to support power drive micro-chips and surface mount
electronic components as an integral part of the printhead. The PZT
elements may be provided with wrap-round electrodes. This both
eliminates the necessity for wire bonding (a higher degree of
integration thereby being obtained) and allows the entire
electrical component to be passivated and/or encapsulated (whereby
the entire assembly is highly protected from chemical attack).
In application, there is inevitably some degree of variation in the
characteristics of individual transducers within an array. This
variability is undesirable as it leads to different performance
characteristics between nozzles. Methods to reduce such variability
between local regions therefore are of benefit. Such methods
include: alteration of the electrode pattern of the excitation
means of the transducer by, for example, selective laser ablation
of the electrode; physical removal of material from the transducer,
particularly of beam regions of the transducer thereby altering the
frequency response of the said beam, by for example the action of
laser light; physical removal of material from the excitation means
by for example, micro-machining.
In a further embodiment (see FIG. 14), transducers, for example
flexural transducers, are provided wherein the width of the
transducer and the corresponding slit width between adjacent
transducers vary along their length. In this embodiment the
transducers 97 are therefore not rectilinear in form, but tapered
toward either end 45, 46. The symmetry of the construction
maintains the condition that the transducers in the array have at
least one common edge in parallel, for example, edges 116 and 117.
The tapers reduce the bending stiffness of the beams continuously
towards the nozzle region which advantageously causes the bending
of the beam to be enhanced in that region, increasing droplet
production efficiency. The nozzle-bearing flexural layer 44,
formed, for example, of piezoelectric ceramic, and bearing the
transducers 97 has thickest width of 169 microns distally from the
nozzles 47 as shown at 45, 46, and thinnest width of 84.5 microns
proximal to the nozzles 47. The interstitial areas 48 between
transducers 97 are sealed by compliant polymer material layer 96,
such as 25 micron thick Upilex; which has the added benefit of
acting to absorb crosstalk emanating from transducer to another
when said transducers are individuals excited into motion. Nozzles
47 are formed through both transducer layers 44 and 96.
In a further embodiment, an example of which is shown in FIG. 15, a
section through a flexural transducer 95 and support layer 53, 54
is illustrated wherein layer 49 is formed of PZT and is
approximately 200 microns in thickness. In operation, voltage
applied to the electroded surfaces (the outer face and inner face
as defined by the passage of nozzle 52 therethrough) causes flexure
of layer 49 substantially parallel and/or anti-parallel to the axis
of nozzle 52, which flexure is enhanced by the introduction of
regions (grooves) thinned by approximately 100 microns, at the
distal ends 50, 51. The spacing between the two thinned regions is
2.0 mm, giving an operating frequency of approximately 90 kHz.
FIG. 16 shows Finite Element modelling results of the device
illustrated in FIG. 15. The graph gives modelling results for 6
devices, each with different thicknesses of the polymer sealing
layer 74, based on a construction which uses Upilex as the material
for this layer. If the thickness of this layer is less than 10
microns, the amplitude of motion of the nozzle is constant at 8.5
microns peak-to-peak. If the thickness of the sealing layer is
greater than 100 microns, the sealing layer damps the nozzle motion
so that the nozzle amplitude is too small to give fluid ejection.
The modelling shows that a layer of 25 micron thick Upilex will be
suitable to seal the slits against fluid egress without inducing
damping or crosstalk.
In the example illustrated in FIG. 15, the transducer is entirely
formed from a single PZT layer, although the principle to be
illustrated may be otherwise embodied according to variants given
within this specification. The transducer 95 is mounted on a
support layer. 53, 54, of, for example, stainless steel, acting to
clamp the distal ends of the local region, and situated in
correspondence to the thinned regions to maximize enhancement of
the flexural motion. Again, the nozzle may be replaced by a simple
aperture, and layer 49 may be overlaid by a further nozzle-bearing
polymer layer 74 (the nozzles in registry with those apertures)
that seals the slits between transducers, acts to protect the outer
face of the layer 49, and provides interconnect.
Such a construction is illustrated in FIG. 17, employing a
multilayer structure, wherein the flexural members 55 of the
transducer 59 are divided in two parts, the nozzle region 56, with
a nozzle therein 57, being formed in a separate sealing layer 58.
The separate layer 58 serves to provide both a substrate for nozzle
formation and a means of sealing between transducer elements. Such
a allows the nozzle to form the largest transverse dimension within
the structure and allows the slits to have width that is a
significant fraction of the diameter of the nozzle. A further
advantage of this realization is the larger damping effect for the
reduction of crosstalk when a flexible polymer is used for the
separate layer 58 with wider separation between transducers than
attainable by a simple slit.
In FIG. 18, it is illustrated how a modification to boundary
conditions at the distal portion of the transducers 72, 73
separated by slits, 74, 75, 76, may be afforded by the inclusion of
a further stiffening layer 77. The layered construction of the
micro-droplet deposition apparatus uniquely allows the optical
alignment of this further stiffening layer. The further layer may
be arranged such that tabs 78, 79 underlie their corresponding
transducer components 80, 81 thereby stiffening the hinged or
clamped join at the area of overlap 82, consequentially allowing
the flexural layer 83 to be formed to be of comparatively lower
stiffness than otherwise possible. The stiffening layer also
advantageously prevents crosstalk between distal sections of the
local regions by forming an acoustic barrier between them.
In FIG. 19, an embodiment is shown, comprising a section of a
transducer 89, bearing a nozzle 90, in which the action of the
further stiffening layer is achieved by the support layer 84, which
also acts to contain the fluid 85. The excitation means 86 , in the
illustration, overlies the material layer 87 in which the local
regions are formed, and the excitation means is so placed that its
distal end 88 overlies the support layer 84, thereby achieving the
modification of the boundary constraint at the distal ends of the
transducer substantially to the hinged condition.
A final embodiment is shown in FIG. 20, wherein the liquid
projection apparatus is configured in a manner suitable for digital
printing. The apparatus is constructed on a material layer 59, on
which a two-dimensional array of transducer components are arranged
in a number of lines 60. A detail of the transducer geometry 61 is
shown in the inset 62 for clarity. The transducers are formed with
a nozzle 63, excitation means 64 and are separated from one another
by slits 65. In this embodiment the transducers are shown with two
slits per transducer, in contrast to previously shown embodiments,
although one slit could also be employed. The transducer array as
shown is advantageously arranged such that the lower fabrication
resolution spacing 66 is perpendicular to the print direction 67.
The printing resolution of each line is thereby maximized to the
spacing 68. The additional lines of the array 60, are staggered at
a fraction of the spacing 68 (at one quarter of the spacing 68 in
the case shown), and so allows the printing resolution to be
further enhanced 69 by overlaying the print from separate lines
successively. A final elaboration shown in this embodiment is the
arrangement of a number of transducers on a sub-array 70, at an
angle 71 to the lines of the transducers 60. The arrangement 70
allows the printing signal to the neighbouring transducers to be
delayed in time with respect to the other transducers in the
sub-array, so that any remaining crosstalk between neighbouring
transducers are distributed in time. There are many permutations
that could be envisaged in the relative placement of transducers
within the sub-array, and the embodiment shown herein is only one
example of such permutations.
A schematic layout of the electronic drive for the operation of the
liquid projection apparatus is shown in FIG. 21. A personal
computer 111 is shown running suitable software such as ETC M321
Generator Software produced by ETC s.r.o., Zilina, Slovak Republic
which provides data to a corresponding drive card 112 such as a ETC
M321 Generator card from the same supplier. The signals so produced
are delivered via a custom made amplifier 113 to liquid projection
apparatus 114 as described in the text. The drive signal is shown
schematically in FIG. 22 wherein an the example wave-form 115 is
illustrated. The typical peak voltage of this waveform is between
40 and 150 V.
* * * * *